The present invention relates, in general, to a linear drive for adjusting a movable structure, in particular a furniture structure.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Linear drives of a type involved here are especially used for adjusting a structure of a furniture item, e.g. slatted frame or armchair, and are available as so-called single drives with a spindle drive and a drive motor, or as so-called double drives with two spindle drives which can be operated by a motor or by two motors. The spindle of each spindle drive is normally operated by a d.c. motor via a speed reduction gear train. Screw drives are typically used for reducing the motor speed.
Linear drives are low-cost mass products. The spindle of each spindle drive has attached thereon a lifting nut which is secured against rotation. The spindle is normally made of steel, while the lifting nut is made of plastic. The lifting nut is considered an expendable part which generally wears off after exceeding a certain operating time or after extended exposure to an excessive force. As a result, the structure to be adjusted suddenly collapses or moves in the direction of the applied load. This is uncomfortable for the user. Therefore, the use of a safety nut has been proposed for attachment onto the spindle to provide a stop by which the lifting nut is prevented from uncontrollably traveling along the spindle. While this approach allows the attached structure to be lowered at relatively small speed, the attachment of such a safety nut does no longer permit a lifting of the structure and thus operation of the structure in opposition to the applied load direction. Therefore, normal operation can resume only after replacement of the lifting nut.
Proposals to connect the safety nut with the lifting nut via webs to prevent continuous operation with the safety nut by rupturing the webs when applying a lifting force have proven impractical because in addition to the necessary replacement of the lifting nut, it is also required to replace the safety nut. Another approach to provide a coupling device between the lifting nut and the safety nut has also proven impractical in view of the complicated construction.
It would therefore be desirable and advantageous to provide an improved linear drive to obviate prior art shortcomings.